This invention relates to optical power meters and optical receivers and more particularly to optical power meters and receivers which are combined for simultaneous operation.
Most prior art optical power meters use a single photodiode in combination with an integrating transimpedance amplifier to provide an output voltage which is representative of the average input optical power. Such optical power meters are characterized by high sensitivity and low bandwidth. Similarly, most prior art optical receivers use a single photodiode in combination with a transimpedance amplifier to provide a voltage output which is representative of the instantaneous current through the photodiode. Such optical receivers are characterized by lower sensitivity and high bandwidth.
A prior art optical power meter is shown in FIG. 1. The optical power meter 10 consists of an operational amplifier 18 in which resistors 12 and 24 form current summing inputs at the positive and negative inputs of operational amplifier 18. Oftentimes resistors 12 and 24 are actually several resistors in parallel which are switched into the circuit to provide alternative gain modes. The input current is provided by photodiode 16 in response to an external light source. Integrating capacitors 14 and 22 integrate the output voltage which is provided at terminal 20 and is representative of the average input optical power. The prior art optical power meter of FIG. 1 does not apply any bias voltage across photodiode 16. In the zero bias condition, photodiode 16 has the greatest sensitivity since minimal leakage (also known as "dark" current) is produced. However, in the zero bias condition, the capacitance of the photodiode 16 is relatively large compared with the capacitance in the reverse biased condition. This, in conjunction with the integrating capacitors 14 and 22, limits the effective bandwidth of the optical power meter 10.
A prior art optical receiver is shown in FIG. 2. The optical receiver 26 consists of an operational amplifier 38 in which resistor 34 forms a current input at the negative input of operational amplifier 38. The input current is provided by photodiode 32 in response to an external light source. A reference voltage, V.sub.REF, is provided to apply a bias voltage across photodiode 32. A small value resistor 28 and a small value capacitor 30 provide a slight frequency compensation which filters out high frequency transients. An output voltage is produced at terminal 36 which is representative of the instantaneous current through photodiode 32. The prior art optical receiver of FIG. 2 maintains a bias voltage of approximately V.sub.REF across photodiode 32. In the biased condition, photodiode 32 has reduced capacitance, since junction capacitance is a function of voltage. However, in the biased condition, a leakage or dark current is produced and is shown as I.sub.L. While the frequency response of the optical receiver shown in FIG. 2 is improved over that of the optical power meter of FIG. 1, the sensitivity is diminished. The diminished sensitivity is due to the shot noise which is associated with the leakage current, I.sub.L, the reduced value of resistor 34, necessary for high speed operation, and the nature of operational amplifier 38 which is maximized for speed and not sensitivity, and therefore has high input bias currents.
Attempts to combine the two prior art circuits have in the past produced circuits in which either the optical power meter suffers from reduced sensitivity or the optical receiver suffers from reduced bandwidth, or both. What is desired is a circuit having a minimum of components which combines an optical power meter and receiver for simultaneous operation in such manner that the high sensitivity of the optical power meter and high bandwidth of the optical receiver are both retained.